Conventional methods of location survey for driving a shield machine used in the shield tunneling method include: a method in which the inside of a tunnel is surveyed with a transit or the like; and a method in which an optical oscillating device for generating a coherent light such as a laser beam is provided inside a vertical shaft of a shield machine, and in which the device emits the light along the planned tunnel line, and then the light spot on a target provided in the shield machine is read, and thus displacement and deflection angle of the shield machine are measured. In still another method, the position of the machine relative to a reference position is obtained with combinations of a direction gyro and a pressure-type subsidence recorder, and of a clinometer and driving distance recorder (referring to length of a segment). Further, the inventor has proposed (in Japanese Patent Laid-Open No. 62-169012) a method in which a magnetic field generating cable is provided on the ground surface and a magnetic field detecting element is provided in an underground excavator, and in which a magnetic field generated from the cable is detected by the detecting element and thus displacement in the horizontal position of the underground excavator is measured.
However, in a case where the inside-tunnel survey method employing a transit or the like is used to survey a to-be-bored tunnel which is curved or bent, many station points are required, increasing the number of work stages. Thus, such a method is not practical. If the method employing a laser beam is used to survey a curved or bent tunnel, the laser beam may not reach a target in some cases. In such cases, an optical oscillating device has to be moved to an appropriate position. Further, if a planned tunnel line is curved or bent, because, in such a case, a light beam can not directly irradiate the planned line, the displacement and deflection angle of the shield machine are usually obtained by measuring distances and angles between the target, the oscillating device and the planned tunnel line and by calculating from such measurements the positions thereof relative to one another. Therefore, extra work stages are required for the transposition of an oscillating device, the measurement and the calculation, and thus excavating efficiency decreases. A method employing a gyro has a problem in that because accumulated error becomes large, it is not suitable for a long-distance excavation. Further, a method employing a magnetic field has a problem in that though it measures horizontal displacement, it cannot measure excavating distance (such as a distance of excavation from a vertical shaft).
It is an object of the present invention to solve the above-mentioned problems by providing a position measuring apparatus of an underground excavator which easily and accurately measures the position on a horizontal plane or the like of a shield machine or the like which is excavating underground.